A typical computer system includes at least a microprocessor and some form of memory. The microprocessor has, among other components, arithmetic, logic, and control circuitry that interpret and execute instructions necessary for the operation and use of the computer system. FIG. 1 shows a typical computer system (10) having a microprocessor (12), memory (14), integrated circuits (ICs) (16) that have various functionalities, and communication paths (18), i.e., buses and wires, that are necessary for the transfer of data among the aforementioned components of the computer system (10).
Circuit elements in a microprocessor (12), and more generally, an IC (16), continue to get smaller. Accordingly, more and more circuit elements may be packed into an IC (16). In FIG. 2, a cross sectional diagram of a particular type of circuit element, a p-channel transistor (200), is shown. The p-channel transistor (200) includes two n+ regions (204, 206) implanted in a p-substrate (210) or a p-well. The two n+ regions (204, 206) form a drain and source region for the p-channel transistor (200). The depth of the drain and source regions may determine a junction (209) thickness for current to flow from one n+ region (204 or 206), through a channel formed below a gate (202) when the transistor (200) is “on,” to the other n+ region (206 or 204). A source contact (212) and drain contact (214) allow a connection with the n+ regions (204, 206), respectively. The p-channel transistor (200) is separated from other devices by a field oxide (230, 232).
The p-channel transistor (200) is controlled by a voltage potential on a gate (202). A gate contact (216) allows a connection with the gate (202). The gate (202) is separated from the p-substrate (210) by a gate oxide (208).
A voltage potential difference between the source contact (212) and drain contact (214) is denoted Vds. A voltage potential difference between the gate contact (216) and the source contact (212) is denoted Vgs. The voltage potential to turn the p-channel transistor (200) “on,” i.e., allow the p-channel transistor (200) to conduct current, is a threshold voltage potential denoted Vt.
FIG. 3 shows a diagram of a current-voltage characteristic for a typical metal-oxide transistor. As shown in FIG. 3, the p-channel transistor (200 shown in FIG. 2) is “off” when |Vgs|<|Vt| (255). The p-channel transistor (200 shown in FIG. 2) is “on” and in a linear region of operation when |Vds|≦|Vgs−Vt| and |Vgs|≧|Vt| (265). The p-channel transistor (200 shown in FIG. 2) is “on” and in a saturation region of operation when |Vds|>|Vgs−Vt| and |Vgs|≧|Vt| (27
As circuit elements in an IC (16 shown in FIG. 1) continue to get smaller, features of the circuit elements, e.g., the gate oxide thickness, the depth of the two n+ regions (204, 206 shown in FIG. 2), the spacing between the two n+ regions (204, 206 shown in FIG. 2), etc., get smaller.